In the exploration of oil, gas, and geothermal energy, wells or boreholes in the earth are created in drilling operations using various types of drill bits. These operations typically employ rotary and percussion drilling techniques. In rotary drilling, the borehole is created by rotating a drill string having a drill bit secured to its lower end. As the drill bit drills the well bore, segments of drill pipe are added to the top of the drill string. While drilling, a drilling fluid is continually pumped into the drilling string from surface pumping equipment. The drilling fluid is transported through the center of the hollow drill string and through the drill bit. The drilling fluid exits the drill bit through one or more nozzles in the drill bit. The drilling fluid then returns to the surface by traveling up the annular space between the well bore and the outside of the drill string. The drilling fluid transports cuttings out of the well bore as well as cooling and lubricating the drill bit.
The type of drill bit used to drill the well will depend largely on the hardness of the formation being drilled. One type of rotary rock drill is a drag bit. Early designs for a drag bit included hardfacing applied to various portions of the bit. Currently, designs for drag bits have extremely hard cutting elements, such as natural or synthetic diamonds, mounted to a bit body. As the drag bit is rotated, the cutting elements form the bottom and sides of the well bore.
Another typical type of rotary drill bit is the tri-cone roller drill bit that has roller cones mounted on the body of the drill bit, which rotate as the drill bit is rotated. Cutting elements, or teeth, protrude from the roller cones. The angles at which the roller cones are mounted on the bit body determine the amount of “cut,” or “bite” of the bit with respect to the well bore. As the roller cones of the drill bit roll on the bottom of the hole being drilled, the teeth or carbide inserts apply a high compressive and shear loading to the formation causing fracturing of the formation into debris. The cutting action of roller cones comprises a combination of crushing, chipping and scraping. The cuttings from a roller cone drill bit typically comprise a mixture of chips and fine particles.
Yet another type of rotary drill bit is a hybrid drill bit that has a combination of hard cutting elements, such as natural or synthetic diamonds and roller cones mounted on the body of the drill bit.
There are two general types of roller cone drill bits; TCI bits and steel-tooth bits. “TCI” is an abbreviation for Tungsten Carbide Insert. TCI roller cone drill bits have roller cones having a plurality of tungsten carbide or similar inserts of high hardness that protrude from the surface of the roller cone. Numerous styles of TCI drill bits are designed for various types of formations, in which the shape, number and protrusion of the tungsten carbide inserts on the roller cones of the drill bit will vary, along with roller cone angles on the drill bit.
Steel-tooth roller cone drill bits are also referred to as milled-tooth bits because the steel teeth of the roller cones are formed by a milling machine. However, in larger bits, it is also known to cast the steel teeth and, therefore, “steel-tooth” is a better reference. A steel-tooth roller cone drill bit uses roller cones, with each cone having an integral body of hardened steel with teeth formed on the periphery. There are numerous styles of steel-tooth roller cone drill bits designed for formations of varying hardness in which the shape, number and protrusion of the teeth will vary, along with roller cone angles on the drill bit.
The cost efficiency of a drill bit is determined by the drilling life of the drill bit and the rate at which the drill bit penetrates the earth. Under normal drilling conditions, the teeth of the steel-tooth roller cone drill bits are subject to continuous impact and wear because of their engagement with the rock being drilled. As the teeth are worn away, the penetration rate of the drill bit decreases causing the cost of drilling to increase.
To increase the cost efficiency of a steel-tooth roller cone drill bit or a hybrid drill bit having steel-tooth roller cones, it is necessary to increase the wear resistance of the steel teeth. To accomplish this, it is known to deposit one or more layers of a wear-resistant material or “hardfacing” to the exposed surfaces of the steel teeth. Fusion hardfacing refers to a group of techniques that apply (fuse) a wear-resistant alloy (hardfacing) to a substrate metal. Common hardfacing techniques include arc welding and gas torch welding, among other welding processes.
Conventional welding techniques used to apply hardfacing to steel-tooth roller cone drill bits include oxyacetylene welding (OAW) and atomic hydrogen welding (AHW). Currently, manual welding is typically used in the commercial production of roller cone rock bits. Roller cones are mounted on a positioning table while a welding torch and welding rod are used to manually apply hardfacing to portions of each tooth of each roller cone by a welder moving from tooth to tooth and cone to cone from various positions.
Conventional hardfacing materials used to add wear resistance to the steel teeth of a roller cone drill bit include tungsten carbide particles in a metal matrix, typically cobalt or a mixture of cobalt and other similar metals. Many different compositions of hardfacing material have been employed in the rock bit field to achieve wear-resistance, durability and ease of application. Typically, these hardfacing materials are supplied in the form of a welding rod, but can be found in powder form for use with other types of torches.
The physical indicators for the quality of a hardfacing application include uniformity, thickness, coverage, porosity, and other metallurgical properties. Typically, the skill of the individual applying hardfacing determines the quality of the hardfacing. The quality of hardfacing varies between drill bits as well as between the roller cones of a drill bit, and individual teeth of a roller cone. Limited availability of qualified welders has aggravated the problem because the application of hardfacing is extremely tedious, repetitive, skill-dependent, time-consuming, and expensive. The application of hardfacing to roller cones is considered the most tedious and skill-dependent operation in the manufacture of a steel-toothed roller cone drill bit. The consistency of the application of hardfacing to a drill bit by a skilled welder varies over different portions of the drill bit.
To summarize, manually applying hardfacing to a roller cone involves the continuous angular manipulation of a torch over the roller cone, the roller cone held substantially stationary, but being rotated on a positioning table. After hardfacing is manually applied to a surface of each tooth of the roller cone using a torch and welding rod containing the hardfacing material, the positioning table and cutter are indexed to a new angle and position to permit application of hardfacing to a surface of the next tooth of the roller cone until all the cutters have been rotated 360 degrees. At that time, the angle of the table and cutter is adjusted for the application of hardfacing to another tooth surface or row of teeth of the roller cone.
When attempts to utilize robotics to automate the welding process were made, the same configuration was used having a robotic arm to replace the human operator's arm and its varied movements, while leaving the roller cone on a positioning table. The positioning table is capable of automatic indexing between teeth and rows of teeth of a roller cone.
This configuration and procedure would be expected to provide the recognized benefits of manual hardfacing for a number of reasons. First, manual and automatic torches are much lighter and easier to continuously manipulate than the heavy steel cutters with teeth protruding in all directions. Second, the roller cone must be electrically grounded, and this can be done easily through the stationary positioning table. Third, gravity maintains the heavy roller cone in position on the positioning table. Fourth, highly angled (relative to vertical) manipulation of the torch allows access to confined spaces between teeth of the roller cone and is suited to the highly articulated movement of a robotic arm.
U.S. Pat. No. 6,392,190 provides a description of the use of a robotic arm in hardfacing of roller cones, in which the torch is held by a robotic arm and the roller cones are moved on a positioning table. A manual welder is replaced with a robotic arm for holding the torch. The robotic arm and a positioning table are combined to have more than five movable axes in the system for applying hardfacing. However, U.S. Pat. No. 6,392,190 does not describe details of solutions to the numerous obstacles in automating the hardfacing of roller cones using robotic arms and positioners.
One factor limiting use of robotic hardfacing has been the unsatisfactory appearance of the final product when applied using robotically held torches over stationary cutters. Another factor limiting use of robotic hardfacing to rolling cutters is the commercial unavailability of a material that directly compares to conventional Oxygen Acetylene Welding (OAW) welding rod materials that can be applied with commercially available Plasma Transferred Arc (PTA) torches.
Another factor limiting use of robotic hardfacing is the inability to properly identify and locate individual roller cone designs within a robotic hardfacing system. The roller cones of each size of drill bit and style of drill bit are substantially different, and initiating the wrong program could cause a collision of the torch and part, resulting in catastrophic failure and loss. Another factor limiting use of robotic hardfacing is the inability to correct the critical positioning between the torch and roller cone in response to manufacturing variations of the cutter, wear of the torch, and buildup of hardfacing.
Still another factor limiting use of robotic hardfacing has been the inability to properly access many of the areas on the complex surface of a roller cone that require hardfacing with commercially available Plasma Transferred Arc (PTA) torches large enough to permit application of the required material. A small form factor (profile) is required to access the roots of the teeth of a roller cone that are close together. However, most conventional PTA torches require large powder ports to accommodate the flow of the medium-to-large mesh powder required for good wear resistance. Torches with smaller nozzles have smaller powder ports that prohibit proper flow of the desired powders.
Another factor limiting use of robotic hardfacing is the complexity of programming a control system to coordinate the critical paths and application sequences needed to apply the hardfacing. For example, undisclosed in the prior art, the known torch operating parameters, materials, application sequences, and procedures used for decades in manual hardfacing operations have proven to be mostly irrelevant to robotic hardfacing of roller cones. A related factor limiting use of robotic hardfacing is the cost and limitation of resources. A significant investment and commitment of machine time are required to create tests, evaluate results, modify equipment, and incrementally adjust the several operating parameters, and then integrate the variations into production part programs. These and several other obstacles have, until now, limited or prevented any commercial practice of automated hardfacing of roller cones.
Therefore, there is a need to develop a system and method for applying hardfacing to roller cones consistent with the highest material and application quality standards obtainable by manual welding. There is also a need to develop a system that identifies parts, selects the proper program, and provides programmed correction in response to manufacturing variations of the roller cones, wear of the torch, and buildup of hardfacing. There is also a need to develop a PTA torch design capable of accessing more of the areas on a roller cone's cutter that require hardfacing. There is also a need to develop a hardfacing material, the performance of which will compare favorably to conventional Oxygen Acetylene Welding (OAW) materials and flow properly through the PTA torch design.